The invention disclosed and claimed herein generally pertains to transfer drive apparatus, that is, apparatus for driving workpiece transfer mechanisms in transfer machines or the like, wherein the apparatus includes a screw and nut mechanism as a principal power transmission element. More particularly, the invention pertains to such transfer drive apparatus wherein a single ball screw operates to simultaneously move different workpieces through distances which are substantially different. Even more particularly, the invention pertains to a transfer drive apparatus of the above type which includes a torque overload disconnect device to protect the ball screw and motor.
In one very useful type of transfer mechanism for a transfer machine, one or more transfer bars are moved forwardly and backwardly by a transfer drive, one forward and one backward movement taking place during each cycle in a succession of transfer cycles. Workpieces are engaged by the bars at various points along their lengths prior to forward movements, and are disengaged therefrom prior to backward movements. Thus, each of the workpieces is progressively moved along a succession of work stations of the transfer machine and processed thereby. A workpiece transfer mechanism of this type is shown, for example, in U.S. Pat. No. 3,361,250, entitled "Mechanical Transfer", which issued Jan. 2, 1968, to Mr. Guy Donald Pierce. Such patent is assigned to The Cross Company, assignee of the present application.
In transfer mechanisms of the above type, the transfer drive must often be capable of moving the transfer bars so that the accelerations and velocities thereof closely follow specified profiles during successive forward and backward movements, or strokes. For example, it may be very desirable or necessary to accelerate transfer bars from zero velocity to a maximum velocity in a very short time at the beginning of each forward and backward stroke, and to thereafter drive the bars at a constant velocity until just before the ends of the strokes. Thereupon, the bars are decelerated so that their velocities reach zero just when the strokes conclude.
By employing a DC electric or other type of variable speed motor as the power source in a transfer drive, the accelerations and velocities of respective transfer bars can be closely controlled, by controlling rotation of the output shaft of the motor. In turn, rotation of the output shaft may be controlled by regulating the electrical input to the motor. Usefully, the motor rotates a ball screw which engages a ball nut immovably joined to a carriage structure or the like, which is mounted for reciprocating or translational movements. The transfer bars may be directly linked to the carriage structure, so that they will move backward and forward with the same velocity and acceleration which is imparted to the carriage structure and the ball nut by the ball screw.
Frequently, the transfer mechanism of a transfer machine must be capable of moving different workpieces over distances which are quite different from one another during the same transfer cycle. Consequently, a transfer mechanism may be provided with two transfer bars, or pairs of bars, which are driven so that their respective displacements during the transfer cycle are likewise different. Through his invention, the inventor provides a transfer drive for such transfer mechanisms, wherein the drive includes a DC electric or other type of variable speed motor as the power source, and further includes a screw and nut mechanism as a key power transmission element. Usefully, a disconnect feature is provided for preventing a torque overload from being placed upon the mechanism, which may comprise a ball screw and associated ball nuts.